Solid-state light emitting diode (LED) image display devices are of great interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic or inorganic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the thin-film material. Different materials emit different colors of light and may be employed in different light-emitting elements to form a full-color display. However, as the display is used, the materials in the light-emitting elements deteriorate and become less efficient at emitting light. This deterioration reduces the lifetime of the display. The differing materials may age at different rates, causing differential color aging and a display whose white point varies as the display is used. In alternative LED displays, a common white-light emitter is used for all light-emitting elements and a color filter supplied with each light-emitting element to provide a full-color display. In this case, each light-emitting element may age at the same rate, if all of the differently colored light-emitting elements are used equally.
Referring to FIG. 7, a graph illustrating the typical light output of a prior-art OLED display device as current is passed through the OLEDs is shown. The three curves represent typical change in performance of red, green and blue light emitters over time. As can be seen by the curves, the decay in luminance between the differently colored light emitters is different. Hence, in conventional use, with no aging compensation, as current is applied to each of the differently colored OLEDs, the display will become less bright and the color, in particular the white point, of the display will shift.
A variety of methods for measuring or predicting the aging of the OLED materials in displays are known in the art. For example, U.S. Pat. No. 6,456,016 issued Sep. 24, 2002 to Sundahl et al., titled “Compensating Organic Light Emitting Displays” relies on a controlled increase in current provided at an early stage of device use followed by a second stage in which the display output is gradually decreased. U.S. Pat. No. 6,414,661 entitled “Method And Apparatus For Calibrating Display Devices And Automatically Compensating For Loss In Their Efficiency Over Time” issued Jul. 2, 2002 to Shen et al, describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes (OLEDs) in an OLED display device, by calculating and predicting the decay in light output efficiency of each pixel, based on the accumulated drive current applied to the pixel; and derives a compensation coefficient that is applied to the next drive current for each pixel. US Published Patent Application No. 2002/0167474 “Method Of Providing Pulse Amplitude Modulation For OLED Display Drivers”, published Nov. 14, 2002 by Everitt describes a pulse width modulation driver for an organic light emitting diode display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light emitting diode in a video display. The voltage driver may receive voltage information from a compensation table that accounts for aging, column resistance, row resistance, and other diode characteristics. U.S. Pat. No. 6,995,519 entitled, “OLED Display with Aging Compensation” issued Feb. 7, 2006 to Arnold et al. describes measuring the voltage across each OLED in a display to produce feedback signals that may be employed to calculate a compensation signal to compensate for changes in the output of each OLED.
U.S. Pat. No. 6,504,565 entitled, “Light-Emitting Device, Exposure Device, And Image Forming Apparatus”, issued Jan. 7, 2003 to Narita et al describes a light-emitting device which includes a light-emitting element array formed by arranging a plurality of light-emitting elements, a driving unit for driving the light-emitting element array to emit light from each of the light-emitting elements, a memory unit for storing the number of light emissions for each light-emitting element of the array, and a control unit for controlling the driving unit based on the information stored in the memory unit so that the amount of light emitted from each light-emitting element is held constant.
JP 2002/278514 A entitled, “Electro-Optical Device” and published Sep. 27, 2002 by Koji describes a method in which a prescribed voltage is applied to organic EL elements by a current-measuring circuit and the current flows are measured. A temperature measurement circuit estimates the temperature of the organic EL elements.
Referring to FIG. 8, prior-art systems providing aging compensation to OLED displays typically include a display 30 for displaying images. The display 30 is controlled by a controller 32 that receives image or data signals 34 from an external device. The image or data signals 34 are converted into the appropriate control signals 36 using conversion circuitry 38 within the controller 32 and applied to the display 30. A performance attribute of the display, for example, the current or voltage within the display 30, is measured and a feedback signal 40 is supplied through a measurement circuit 42 and provided to the controller 30. The controller then uses the measured feedback signal 40 to change the control signals 36 to compensate for any aging detected in the display 30.
The measurement circuit 42 may be incorporated into the display 30, into the controller 32, or may be a separate circuit 42 (as shown). Likewise, the feedback signal may be detected within the display (as shown) or measured externally by the controller 32 or some other circuit. For example, the luminance of the display 32 may be measured by an external photo-sensor or camera or be detected by photosensors on the display itself.
In some prior art embodiments, the feedback signal 40 is not produced by the display 30, but is produced by analyzing the control signals 36 input to the display 30. For example, a useful feedback signal known in the prior art is the accumulation of current provided to the display 30. Since aging depends on total current passed through a display, a measurement of the accumulated current can be used to predict the aging of the display 30. Alternatively, the luminance signal sent to the display 30 as part of the control signals 36 may be accumulated over time to provide the feedback signal 40. A knowledge of the intended luminance of the display 30 can be used to predict aging and then the effects of aging can be compensated. Although a continuous compensation of aging is possible in some of these configurations, compensations are often applied periodically so as not to interfere with the use of the device.
In another aging compensation method described in U.S. Pat. 7,161,566 entitled, “OLED Display with Aging Compensation” issued Jan. 9, 2007 to Cok, a current-measuring device is employed to sense the current used by the display device to produce a current signal that is employed to calculate a compensation signal.
It is preferable that any changes made to the display be imperceptible to a user and represent the best possible compromise between brightness, color, and power usage. Since displays are typically viewed in a single-stimulus environment, slow changes over time are acceptable, but large, noticeable changes are objectionable. Since continuous, real-time compensations are usually not practical because they interfere with the operation of the LED device, most changes in LED device compensation are done periodically. Hence, if an LED device output changes significantly during a single period, a noticeably objectionable compensation to the appearance of the display may result. US Application Publication 2005/110728 entitled, “A Method of Aging Compensation in an OLED Display” by Cok describes a method for controlling aging compensation in an OLED display having one or more light-emitting elements comprising the steps of periodically measuring the change in display output to calculate a compensation signal; restricting the change in the compensation signal at each period; and applying the compensation signal to the OLED display to effect a compensation in the display output. This technique reduces the perceptibility of compensation changes in an OLED display, but does not address the tradeoff between luminance, power, and lifetime inherent in such an OLED display.
All of the methods described above change the output of the LED display to compensate for changes in the LED light-emitting elements. The various prior-art aging compensation methods described above are useful in providing compensation, but do not teach methods for controlling or limiting the compensation. While maintenance of a display at a given luminance and white point is a useful goal, in many applications an alternative compensation may be preferred.
Applicants have found that the maintenance of an LED device at a constant luminance may be impractical. For most display devices, it is preferred to initially employ the device at a maximum luminance, but this maximum luminance may not be required throughout the lifetime of the display device. Moreover, in some circumstances, a minimum power requirement may be preferred. Since increasing the luminance of an LED device requires increasing the power employed, other factors, such as battery lifetime in a portable electronic display device (for example, cell phones, personal video or audio players, and the like) may become more critical performance attributes than the luminance of the display. In any case, there is a limit to the amount of power that a display may dissipate, so that constant luminance may not be achieved.
There is a need, therefore, for an improved aging compensation method for light-emitting diode devices.